1. Field of the Invention
The present invention relates, in general, to direct vessel injection (DVI) nozzles for minimum emergency core cooling (ECC) water bypass and, more particularly, to a DVI nozzle which efficiently injects ECC into a reactor vessel of a pressurized light water reactor (PLWR) to cool a reactor core during a cold leg break (CLB) that may occur in a reactor coolant system of the PLWR, thus remarkably reducing the direct ECC bypass fraction to a broken cold leg and minimizing the amount of direct ECC bypass.
2. Description of the Related Art
Generally, in a nuclear reactor to generate electric energy using a fission reaction, pressurized light water is used as a reactor coolant for carrying the thermal energy generated from the reaction of nuclear fuel. A pressurized light water reactor (PLWR), must be maintained core cooling state during a loss-of-coolant accident to remove the decay heat of core.
In the event of coolant leakage from the reactor coolant system of a PLWR, the reactor core overheats, sometimes causing breakage of the PLWR. In an effort to protect the reactor core against coolant leakage from such a PLWR, emergency core coolant (ECC) is supplied from an outside ECC source to the reactor vessel. In the related art, the ECC is typically injected into the reactor vessel in one of two injection modes: a cold leg injection (CLI) mode and a direct vessel injection (DVI) mode.
Korean Patent Registration No. 10-0319068 discloses a cylindrical reactor vessel 100 having a reactor core 101 therein to generate thermal energy through a fission reaction, as shown in FIG. 1 of the accompanying drawings. A core support 104 is placed in the reactor vessel 100 to support the reactor core 101 in the vessel 100, with a downcomer 105 defined between the reactor vessel 100 and the core support 104. A coolant is introduced into the vessel 100 through cold legs 102 and flows downwards through the downcomer 105 to reach a lower chamber 107 of the vessel 100, and flows to the reactor core 101 to absorb thermal energy from the reactor core 101 and is, thereafter, discharged to the outside of the vessel 100 through hot legs 103.
In the above-mentioned conventional nuclear reactor, direct vessel injection (DVI) nozzles 106 are provided on the vessel 100 at positions adjacent to the cold legs 102 so that the DVI nozzles 106 inject ECC into the vessel 100 to supply ECC to the reactor core 101 in the event of a cold leg break (CLB). Furthermore, safe injection ducts 108 extend from positions around the DVI nozzles 106 to positions around the lower chamber 107 in an effort to prevent injected ECC from being swept into a broken cold leg 102 during a cold leg break (CLB).
However, in the conventional reactor vessel 100 having the DVI nozzles 106 provided at positions adjacent to the cold legs 102, ECC to cool the reactor core 101 during a cold leg break (CLB) may be undesirably swept into a broken cold leg 102 to cause ECC loss. The results caused by the sweep-out of ECC into the broken cold leg 102 are illustrated in FIG. 2. FIG. 2 is a graph illustrating the results of MARS (RELAP5/Mod3 1D) code analysis executed by a computer when ECC provided to protect the reactor core 101 against a cold leg break (CLB) is injected into the vessel 100 in a conventional direct vessel injection (DVI) mode using the DVI nozzles 106 provided on the vessel 100 at positions horizontally offset from the cold legs 102 at 15° angles relative to the cold legs 102 in opposite directions. To calculate a core fuel cladding temperature, the reactor core 101 is divided into twenty vertically arranged volumes and the core fuel cladding temperature is calculated in individual volumes. The twenty volumes are respectively designated by the numbers, Node1, Node2, Node3, Node4, Node5, Node6, . . . Node20, sequentially in order from the bottom to the top of the reactor core 101 so that Node20 designates the top of the reactor core 101. when a cold leg break (CLB) occurs in the reactor coolant system of the vessel 100, some sections of the reactor core 101, namely Node10 to Node15, located within a region from the middle of the reactor core 101 to ⅔ of the way to the top of the core 101, may exceedingly overheat. Variations in the temperatures of four sections of the core 101, which are Node10, Node12, Node14 and Node15, during a cold leg break (CLB) are shown in the graph of FIG. 2. The graph of FIG. 2 shows that the core fuel cladding temperatures become stabilized when two hundred seconds pass after ECC is injected into the vessel 100 to protect the core 101 against the CLB. However, as the DVI nozzles 106 are provided at positions adjacent to the cold legs 102, the core fuel cladding temperatures rapidly increase after four hundred seconds pass after the injection of ECC into the vessel 100. This is so-called “core reheating” that cannot be allowed in the event of such a CLB of a reactor in which all the fuel rods are installed. The core reheating is caused by ECC which does not sufficiently cool the core 101 during the CLB. In other words, the core reheating is caused by an increase in the amount of direct ECC bypass fraction to a broken cold leg 102. Due to the increase in the amount of direct ECC bypass fraction to the broken cold leg 102, the amount of ECC flowing from the downcomer 105 to the reactor core 101 is reduced, while the amount of ECC swept into the broken cold leg 102 increases. When the sweep-out of ECC into the broken cold leg 102 continues, the temperature of the core 101 rapidly increases to cause reactor breakage.